(1) Field of the Invention
The present invention relates to an ignition system for an internal combustion engine, wherein a high DC voltage is applied to a secondary winding of an ignition coil so as to extend the spark discharge for a longer period of time. A high DC voltage is also applied to a capacitor and the capacitive energy charged within the capacitor is sent into one of the spark plugs for sustaining arc discharge at the spark plug. Further, inductive energy in the coil is also sent into the spark plug, in which the spark discharge occurs, upon receipt of a high voltage surge generated at the secondary winding of the coil when a primary current of the ignition coil is interrupted in synchronization with engine rotation.
(2) Description of the Prior Art
A conventional ignition system for an internal combustion engine comprises: (a) a plurality of spark plugs each located within a corresponding engine cylinder; (b) a low DC voltage supply such as a storage battery; (c) an ignition coil; (d) a resistor; (e) a distributor having a rotor electrode and a plurality of fixed electrodes extending radially from the rotor electrode as a center and equally spaced apart from each other, each fixed electrode being connected to the corresponding spark plug via a noise supression cable according to an ignition order; (f) a contact breaker which opens so as to interrupt a primary current flowing through a primary winding of the ignition coil in synchronization with engine rotation; and (g) an arc extinguishing capacitor connected across the contact breaker. The ignition coil has primary and secondary windings, wherein one end of the primary winding is connected to a positive pole of the DC voltage supply via the resistor, one end of the secondary winding is connected to the rotor electrode of the distributor, and the other ends of both primary and secondary windings are connected to each other and grounded via the contact breaker. When contact points of the contact breaker are separated, the primary current flow from the low DC voltage supply through the primary winding of the ignition coil and resistor to ground is interrupted so that a high voltage surge with a peak value of minus 20 kilovolts to minus 30 kilovolts is generated at the secondary winding of the coil. The high voltage surge is sequentially applied to one of the spark plugs during the ignition stroke of the engine cycle via the distributor. At this time, a spark discharge occurs at a discharge gap of the spark plug when the high voltage surge exceeds a breakdown voltage of the gap. Subsequently, inductive energy stored in the ignition coil extends the discharge phenomenon so as to ignite the compressed air-fuel mixture supplied into the corresponding engine cylinder.
However, there are drawbacks in such a conventional ignition system. Since the inductive energy in the ignition coil is sent into the individual spark plugs, the capacity of the ignition coil (inductance) needs to be increased to increase the ignition energy for lean air-fuel mixture. However, since there is a limitation for increasing the capacity of the ignition coil, it is hardly possible to increase the ignition energy immediately after the ignition is started, e.g., during engine cranking. The ignition start has a close connection with fuel economy. On the other hand, if the discharge duration is extended in order to improve combustion stability at the time of engine idling and low engine speed, the capacity of ignition coil needs to be enlarged and consequently the efficiency of supplying ignition energy is decreased.